This invention relates to a startup control method for a passive transponder adapted to be started up in response to an interrogation signal from an interrogator and to return required data back to the interrogator such as a control method that may be applied to a tire pressure monitoring system (TPMS), and in particular to an interrogator in such a system.
Except when used for a particular purpose, vehicular tires are of an air injection type. Thus, whenever a tire of this kind is set to a wheel and attached to a vehicle, its air pressure must be adjusted appropriately according to the type of the vehicle. An inadequate tire pressure causes not only discomfort to the passenger, a reduction in the travel stability and excessive wears to the tires but also the bursting of the worn-out tire at the time of a travel at a high speed in a worst situation where the air pressure is particularly low.
In general, the tire pressure is measured by pressing a dedicated instrument such as a pressure gauge onto the tire valve but such a method can be used only when the vehicle is stopped and can be done only intentionally. In other words, the tire pressure cannot be monitored constantly. Although the checking of tire pressure may be one of the inspection routines and the aforementioned worst condition may be avoided by following the inspection routines, such routines cannot be expected to be followed all the time.
In view of this situation, a law was passed in the United States to make it mandatory to install a tire pressure alarm system to a vehicle. A substantial market expansion in response to this new law is expected to take place towards the end of 2006 but cooperative efforts have already started among the makers of tires, valves and electronic equipments.
The focus of attention is not on the deflation detection systems (DDSs) which have already been installed to some of the vehicles but the tire pressure monitoring systems (TPMSs). Unlike the DDS adapted to indirectly monitor the tire pressure by making use of outputs from a wheel speed sensor used for an anti-lock brake system (ABS), the TPMS is adapted to monitor all tires individually and directly by providing a sensor unit at the valve portion of each tire and hence is capable of an accurate monitoring even while the vehicle is stopped or parked.
Japanese Patent Publication 9-509488 (corresponding to U.S. Pat. No. 6,087,930) has described a technology of TPMS comprising a transponder provided to each tire of a vehicle and an interrogator attached to the body of the vehicle. The interrogator is a device that interrogates and the transponders are adapted to respond by returning a signal in response to an interrogation from the interrogator. The transponders are each provided with a sensor for detecting the conditions of the corresponding tire, but the conditions of a tire are variables related to the tire which change with time, inclusive basically of the tire pressure and also of the temperature and rotary speed of the tire. Thus, the sensor is primarily an air pressure sensor or a compound sensor including a pressure sensor, a temperature sensor and a rotary motion sensor. The conditions of a tire may also include data related to its production although they are not variable data. In such a situation, a memory recording such data may be included in the sensor or its peripheral circuit part.
In addition to such a sensor, the transponder further includes a signal receiver for receiving interrogation signals transmitted from the interrogator and a signal transmitter for transmitting the sensor data from the sensor in response to the interrogation signal received by the signal receiver. In summary, this is a passive apparatus because it starts up in response to an interrogation signal from the interrogator and is adapted to return a response signal.
The interrogator on the vehicle body is provided with a signal transmitter for transmitting a wireless interrogation signal to the transponder of each tire, a signal receiver for receiving response signals (of sensor data, tire pressure data and other data) returned from the transponders in response to this interrogation signal, and a reporting part for making judgment on the various data contained in these response signals (tire pressure data in particular) received by the signal receiver and reporting the result of such judgment to the vehicle operator.
With a prior art system thus structured, the interrogator can make judgments on abnormal conditions of the individual tires on the basis of the response signals returned from the transponders and report on these judgments to the vehicle operator. In other words, the abnormal conditions of variable data such as tire pressure can be monitored constantly and continuously. It is therefore extremely effective in the prevention of worst situations such as the bursting of tires. Since each of the transponders is of a passive kind, starting up only in response to an interrogation signal appropriately transmitted from the interrogator on the vehicle body, wasteful consumption of the inner batteries of the transponders can be prevented because the operations are stopped when the vehicle is being parked over a long period of time or when a tire is being removed from the vehicle.
There are problems with the prior art system as described above. For example, the number of transponders that are required for each vehicle depends on the number of tires on that vehicle. In the case of a four-wheel vehicle, for example, five transponders will be required inclusive of the one for the spare wheel. In order for the interrogator to receive response signals from so many transponders without having them confusingly mixed together, it is necessary to start up the transponders of the individual tires independently. One of the methods for individual startup will be to include ID data in the interrogation signals from the interrogator and to start up only the transponders with matching ID data. Another method will be to set transmission antennas individually near the transponders for interrogation signals and to transmit the interrogation signals from these transmission antennas in time divisions.
The problem with the former method is that the receivers of all transponders and the circuits for judging the matching of ID data must be always switched on. This means that the power consumption of the transponders becomes large and the internally contained batteries are quickly consumed.
The problem with the latter method is that various adjustments are required such that only one transponder will be inside the cover area of each transmission antenna. These adjustments will include the optimum design for each vehicle type (the optimum design of transmission antennas making pairs with the transponders) and the adjustment of power for each transmission antenna. This results in an increase both in production cost and the work cost. Moreover, even if optimum designs and power adjustments are carried out, power may change due to changes over time and in the environmental conditions of the transmission antennas. Thus, the vehicle will have to be regularly brought to a factory or the like for readjustment of power. This affects the use efficiency of the vehicle adversely.